Earthquake Monitoring at Mount St. Helens

Due to the eruptions of 1980-86 and 2004-2008, Mount St. Helens has had the best seismic monitoring network of all volcanoes in the
Cascade Range. It is also the most seismically active volcanoes in the Washington and Oregon
Cascades.
In an average month, 22 events are located by the Pacific
Northwest
Seismic Network (PNSN), with the number going far higher during eruptive periods.
Although a
few seismic stations were installed near Mount St. Helens in the 1970s, the first complete network
of
stations was installed in 1980 in response to unrest starting in March of that year. Since then
millions of
earthquakes, as well as other non-earthquake signals (e.g., rockfalls, explosions, avalanches,
glacier
quakes, helicopters) have been recorded. The PNSN earthquake catalog encompasses periods of
precursory activity (preceding eruptions in 1980-1986 and 2004), seismicity associated with
episodic
explosive eruptions (1980) and dome growth (1980-1986 and 2004-2008), and times of relative
quiet
between eruptions (1987 - 2004 and 2008 to present). Seismic data recorded by this network
have
been used in many studies, including:

forecasting eruptions and detecting explosions

determining eruption dynamics

developing models of the magmatic system beneath Mount St. Helens

determining the on-the-ground processes responsible for various types of seismic signals

detecting repetitive events (or earthquake families), including the first published study of
repetitive
events in a volcanic setting.

1980-1986

One of the intriguing aspects of volcano seismology is trying to determine the cause, volcanologic
or
otherwise, of a particular earthquake. This is illustrated by reviewing a plot of earthquake depths
over
time (right).

The May 18, 1980, eruption is marked by a vertical streak of earthquakes extending down well
below 15
km (9.3 miles). Scientists believe that these earthquakes occurred when the May 18 eruption
drained
magma from deeper parts of the magmatic system, leaving voids of unsupported rock that then
failed
and produced earthquakes.

In contrast, the 1981-1986 time period was dominated by shallow earthquakes, mainly less than
2 km
(1.2 mi) deep. These earthquakes occurred primarily as precursors to individual dome-building
eruptions, and are thought to have occurred as a result of stress accumulations associated with
magma
movement.

1987-2004

Although eruptions stopped after 1986, earthquakes continued to occur, including several years-
long
dense groups (swarms) of "deep" earthquakes (depths greater than 3 km, or 1.9 mi), which hadn't
been
seen since 1980. Scientists think these particular "deep" earthquakes probably occurred when new
magma entered the system from below and caused the pressure to increase on the system. In
contrast,
shallow earthquakes from 1987-2004 probably occurred due to a combination of factors: magma
left-
over from the 1980-86 eruptions cooled, contracted, and created earthquakes; heated
groundwater
(hydrothermal fluids) circulated and added stress to the volcanic structure; and magma potentially
accumulated below 3 km (1.8 mi), which added stress to the system.

2004-2008

The 2004-2008 eruption produced many earthquakes, with well over one million occurring
in
association with the construction of a new lavadome complex. A notable phenomena observed
during
the eruption was the occurrence of very regularly spaced patterns of small earthquakes, which
were
dubbed "drumbeat" earthquakes. These consistently spaced seismic events accompanied the
steady
eruption of lava spines as they emerged from the Mount St. Helens conduit, and have since been
reported at other volcanoes (e.g., Augustine Volcano in 2006).

Owing to the robust seismic data set for Mount St. Helens, scientists understand the character of
typical
eruptive and non-eruptive earthquake patterns at this volcano. These perspectives are vital assets
when
interpreting the significance of earthquake activity at the volcano, and they can help scientists to
determine when a future eruption may occur. The PNSN's
website is an excellent resource for viewing and mapping earthquakes at Mount St. Helens.

Tectonic Earthquakes

In addition to earthquakes that occur due to volcanic processes, seismicity at Mount St. Helens
also
occurs along tectonic faults, which are associated with motion in the crust and not with volcanic
processes. Many of these earthquakes occur along the Mount St. Helens seismic Zone (SHZ), which
extends north-northwestwards to Morton, Washington and south-southeastwards to past the
Swift
Reservoir. Three to four earthquakes per month occur along the SHZ, including a number of
events that
have been large enough to be felt in nearby communities. The largest event ever recorded on the
SHZ (a
magnitude 5.2) located near Elk Lake on February 14, 1981. The most recent felt event was a
magnitude
4.3 on February 14, 2011, that was felt as far away as Vancouver, Washington. In contrast to
earthquakes beneath Mount St. Helens, earthquakes occurring in the area surrounding the
volcano,
including the SHZ, are all thought to be normal "tectonic" earthquakes caused by tectonic forces
that
also produce earthquakes throughout western Washington and northwestern Oregon.